Off-the-Record (OTR) Messaging allows you to have private 
conversations over instant messaging by providing:

* Encryption
       No one else can read your instant messages.
* Authentication
       You are assured the correspondent is who you think it is. 
* Deniability
       The messages you send do not have digital signatures that
       are checkable by a third party. Anyone can forge messages
       after a conversation to make them look like they came from
       you. However, during a conversation, your correspondent is
       assured the messages he sees are authentic and unmodified. 
* Perfect forward secrecy
       If you lose control of your private keys, no previous 
       conversation is compromised.

WWW: http://www.cypherpunks.ca/otr/